So called multiaxis tube benders are finding increasing use as a rapid and precise means for continuously bending a curvature into a metal tube. Such a bender includes a fixed die and an axially spaced movable die, each of which closely surrounds the exterior cross section of the tube. A powerful drive mechanism forces the tube through both dies as the movable dies swings through a preprogrammed set of motions to bend the tube, relative to the fixed die, into a desired curvature. A typical multiaxis bender may be seen in U.S. Pat. No. 4,391,116.
It is desirable to support the tube cross section against deformation, such as buckling or wrinkling, as it is bent. Generally, an interior bending mandrel is used, such as can be seen in U.S. Pat. No. 3,258,956. Interior mandrels were in use for tube bending long before multiaxis benders existed, but they work well with such machines, at least when the tube to be bent is complete on all four sides. However, when the tube is not complete, but open along one side, or U-shaped in cross section, then an interior mandrel will not work, as it cannot be confined inside the open section. The same is true when the "tube" is not a tube in the conventional sense, but rather a long, thin piece of complex cross section, like an I-beam, which also has no defined, enclosed interior.